Question

I am trying to learn C++ and trying to write a code for a simple hash table like following structure:

array[0][0] array[0][1] array[0][2]
key 1        value 1      value 2

array[1][0] array[1][1] 
key 2        value 3     

array[2][0] array[2][1] array[2][2]
key 3        value 4      value 5

means Array of Dynamic Arrays. Now, below is my code:

#include<iostream>
#include<conio.h>

using namespace std;

 int ele [10] ;
 int** arrays = new int*[10] ;

class HashTable
{
   public:

    HashTable()
    {    
         for(int i = 0 ; i < 10 ; i++)
         ele[i] = - 1 ;     // element array stores the number of elements in column of each row
    }

    void put(int key, int value){
         if(ele[key] == -1){
         arrays[key] = new int[1];
         arrays[key][0] = value ;  // initialize 2nd dimention
         ele[key] = 0 ; 
         }
         else{
              int num = ele[key] ;
              int temp[num + 1] ;
              for(int i = 0 ; i < num ; i++)
              temp [i] = arrays[key][i] ;
              temp[num+1] = value ;
              arrays[key] = new int[num + 1] ;
              for(int i = 0 ; i < num+1 ; i++)       // take all the elements in an temporary array and store it back
              arrays[key][i] = temp [i] ;
              ele[key] = num + 1 ;
              }

         }

};

main()
{

HashTable object;

 object.put(0 , 100);
 object.put(1 , 200);
 object.put(3 , 300);
 object.put(3 , 3000);
 object.put(3 , 30000);
 object.put(5 , 500);
 object.put(5 , 5000);
 object.put(5 , 50000);
 object.put(5 , 50); 


for (int i = 0 ; i < 10 ; i++ ){
int j = ele[i] ;

cout << j << " K ";

if(j != -1){
for (int k = -1 ; k < j ; k++ )    // print the values of corresponding keys
cout << arrays[i][k] << "  ";
}
}


 getch();
   return 0;
}

which uses put method to put values in the custom HashTable.

Can anybody help me why above code gives wrong output value while trying to retrieve values of the corresponding key.

Answer 1

This can’t work:

for (int k = -1 ; k < j ; k++ )
    cout << arrays[i][k] << "  ";

You cannot have arrays with negative indexes. When you enter the loop, you have k = -1 so you try to access arrays[i][-1], which won’t work.

Other recommendations:

• The objects arrays and ele should be members of your class and be declared inside it (as private members)
• On the line where you write arrays[key] = new int[num + 1], you forget to call delete on the previous object. This will cause a memory leak.
• You should print your arrays using a method inside your function (like the put method) and then just call it using object.print() (if you named it print)
• Testing if(j != -1) before the loop for (int k = -1; k < j; k++) is redundant: if j == -1 you will never go into the loop anyway.